In order to save a building and human lives from house fire, it is effective to install a fire detector that detects the occurrence of fire at an early stage and gives an alarm. For this purpose, when the fire detector installed in a monitored region detects fire, the fire detector outputs an alarm signal to sound an alarm bell or an alarm speaker, thereby notifying the occurrence of fire.
However, in a building having high sound insulation such as a hotel, even when the alarm bell installed on an access is sounded, the alarm sound is not easily audible by users within living rooms. To solve this inconvenience, a sounder (a base sounder) that is directly fitted to the fire detector within the living room and generates alarm sound based on the output from the fire detector is put into practical use. For example, U.S. Pat. No. 6,362,726 discloses a base sounder that can be fitted to a fire alarm system. According to such base sounder, the alarm sound can be output at the same position as the fire detector within the living room, thereby more securely achieving the fire alarm.
A configuration of the above conventional base sounder is explained. FIG. 17 is a vertical cross-sectional view of the conventional sounders installed on the ceiling surface. As shown in FIG. 17, the conventional base sounder 100 is fitted to a ceiling surface 102 via a fitting base 101. A fire detector 103 is connected to a lower end of the base sounder 100. Electric constituent elements such as a circuit substrate 104 and a piezo element 105 are accommodated inside the base sounder 100. Alarm sound output from the piezo element 105 is discharged to the outside of the base sounder 100.
Output control of the conventional base sounder is explained below. In general, the base sounder using the piezo element 105 for the sound source outputs alarm sound by applying a pulse signal to the piezo element 105. Specifically, plural MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors) (not shown) are combined to configure a driver circuit of a full bridge. A pulse signal having a constant width generated by a pulse switching using this driver circuits is applied to the piezo element 105.
Alarm sound is sometimes desired to be output in different sound volumes and at different pitches according to kinds of alarm and urgency levels. For example, when a fire detector is connected to plural other fire detectors linked to each other, the fire detector outputs alarm sound in a relatively high tone when the fire detector itself has detected fire, and the fire detector outputs alarm sound in a relatively low tone when the fire detector notifies fire detected by other fire detector. In order to output plural alarm sounds, the conventional base sounder simply changes the amplitude and the frequency of a pulse signal applied to the piezo element 105.
Patent Document 1: U.S. Pat. No. 6,362,726